Apparatus and process for the manufacture of optical fiber bragg gratings

ABSTRACT

The present invention is a process for manufacturing an optical fiber Bragg grating, which in a preferred embodiment includes the steps of: (a) removing at least a portion of a removable coating on an optical fiber element in at least one section to sufficiently expose the optical fiber in the section for a subsequent treatment by a source of optical radiation; (b) fixing the at least one section with respect to the source of optical radiation; (c) directing optical radiation from the source into the optical fiber to produce at least one Bragg grating in the at least one section; and (d) covering the at least one section. The present invention also extends to an apparatus for carrying out the process steps described above, which includes means for coating removal, means for fiber immobilization, means for writing a Bragg grating, and means for packaging.

[0001] This application is a Divisional of U.S. patent application Ser.No. 08/735,468, filed Oct. 23, 1996, which was a continuation of U.S.patent application Ser. No. 08/631,491, now issued as U.S. Pat. No.5,939,136 on Aug. 17, 1999.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an apparatus and process forforming a Bragg grating in an optical fiber element. More particularly,the present invention relates to an apparatus and a continuous orstepwise continuous process for making optical fiber Bragg gratings in acoated optical fiber element. In a presently preferred embodiment, theprocess of the present invention includes the steps of removing asufficient amount of a removable coating from at least one section of anoptical fiber element such that optical radiation may access a core ofthe optical fiber, immobilizing the predetermined section of the opticalfiber, forming at least one Bragg grating in the predetermined sectionof the optical fiber, and treating the predetermined section of theoptical fiber to package the Bragg grating.

[0004] 2. Description of Related Art

[0005] A Bragg diffraction grating is a structure that has a periodicpattern of alternating high and low optical refractive index values.Bragg gratings are useful because of their ability to reflect aparticular wavelength or “color” of light. The color that will bereflected by a grating is the color whose wavelength exactly matchestwice the effective grating period. See, for example, Morey et al.,Photoinduced Bragg Gratings in Optical Fibers, Optics and PhotonicsNews, vol. 5, no. 2 (February 1994); Meltz et al., Formation of BraggGratings in Optical Fibers by a Transverse Holographic Method, Opt.Lett. 14 (1989) at 823-25.

[0006] It is well known that Bragg gratings may be formed by creating aninterference pattern in the germanosilicate glass core of an opticalfiber, typically by recombining two parts of the beam of an ultravioletlaser. The first optical fiber Bragg gratings were produced accidentallywhen an argon ion laser remained focused into the end of an opticalfiber for a period of hours. A portion of the beam was reflected backupon itself in the fiber, producing a standing wave interferencepattern. In the bright sections of the interference pattern (where theforward- and backward-traveling waves reinforce each other), the laserlight interacted with germanium sites in the fiber core and changed thelocal refractive index. At the dark sections of the interference pattern(where the two waves destructively interfere and cancel each other), therefractive index remained unchanged.

[0007] However, this “end launch” method of “writing” Bragg gratings inoptical fibers allows almost no control of the location of the gratingwithin the fiber, the angle of the grating planes with respect to theoptical fiber axis, or the grating period. All of these variables areimportant to control when constructing useful devices based on fiberoptic Bragg gratings, and the end launch method has not proved usefulfor producing optical fiber Bragg gratings in commercial quantities.

[0008] To provide greater flexibility in the design of fiber optic Bragggrating devices, techniques have been developed to write gratings byapplying optical radiation through the side of (e.g. normal to thelength of) an optical fiber. One such technique, as illustrated in U.S.Pat. Nos. 4,725,110 and 4,807,950, involves splitting a laser beam intotwo sub-beams and recombining these sub-beams at a known andcontrollable angle within the core of the optical fiber. A secondwell-known technique described in the technical and patent literatureinvolves focusing the laser beam on the fiber core through a grooved orpatterned transmissive optical element known as a phase mask. This phasemask holographically creates an interference pattern in the opticalfiber core.

[0009] The above-described techniques for producing optical fiber Bragggratings are well established, but certain technical difficulties todate have prevented their use in large scale continuous or stepwisecontinuous production processes. For example, a significant productionproblem is removal of the coating which covers the section of theoptical fiber to be treated with the laser. Optical fibers are producedwith a coating which protects the delicate glass structure from chemicalor mechanical attack, and this coating must be substantially completelyremoved if the applied optical radiation is to access and form a Bragggrating in the optical fiber core. If a coated optical fiber is to beused in the manufacture of a fiber Bragg grating, it is necessary firstto thermally, chemically or mechanically remove all or a part of theprotective coating from the coated optical fiber to leave an opticallytreatable, preferably bare, fiber surface. See, for example, Rizvi andGower, Production of Bragg Gratings in Optical Fibers by Holographic andMask Production Methods, The Institute of Electrical Engineers, OpticalFiber Gratings and Their Applications, January 1995.

[0010] However, conventional thermal, mechanical or chemical means forstripping the coating from the bare fiber in manufacturing processes aretime consuming and reduce the physical integrity of the fiber. See,e.g., M. C. Farries et al., Fabrication and Performance of PackagedFiber Gratings for Telecommunications, The Institute of ElectricalEngineers, Optical Fiber Gratings and Their Applications, January 1995;Tang et al., Annealing of Linear Birefringence in Single-Mode FiberCoils: Application to Optical Fiber Current Sensors, Journal ofLightwave Technology, vol. 9, No 8, August 1991. Therefore, carefulremoval of the optical fiber coating is required to form a sufficientlyclean glass surface to allow treatment of the optical fiber core withthe laser, as well as an optical fiber which retains its strength afterformation of the Bragg grating in the core. Time-consuming and laborintensive coating removal steps have to date limited manufacture ofoptical fiber Bragg gratings to production in small batches. In thesebatch processes the coating is typically chemically removed from a shortlength (referred to herein as a “section”) of several optical fibers.The fibers are then treated, one at a time, with a laser using a phasemask projection technique to form Bragg gratings in the sections of theoptical fibers where the coating was removed. These production processesprovide good control over formation of a single Bragg grating in a shortlength of optical fiber. However, the batch technique is obviously noteconomically feasible for large scale Bragg grating production, or forproduction of multiple Bragg gratings in a long length of optical fiberfor grating arrays. In addition, in the batch technique the bare opticalfiber is exposed for significant lengths of time, which may degradefiber strength. To monitor grating quality, the batch technique requiresa termination for each optical fiber end.

[0011] To address the coating removal problems in the batch productiontechnique, some optical fiber Bragg gratings have been written as theoptical fiber is produced on the draw tower. Draw tower production makescoating removal unnecessary, since the optical fiber cores are treatedwith optical radiation to form Bragg gratings before their protectivecoating(s) is applied. Formation of Bragg gratings during fiber drawincreases production volume compared to the batch process describedabove. However, as the optical fibers are drawn on the draw tower, theBragg gratings must be formed with a single shot from the laser, and thedraw process cannot be stopped or interrupted to use different gratingwriting techniques. Further, the Bragg condition (for example, centerband wavelength) of the Bragg grating depends on the exact placement ofa section of the optical fiber relative to a writing zone, and since theposition of the optical fiber drawn on the tower cannot be preciselycontrolled, the grating writing process cannot be sufficiently stablefrom shot to shot. The variation in draw speed also makes preciselocation of the Bragg grating difficult. Therefore, while the draw towerproduction technique increases production speed compared to the batchprocess, this speed comes at a significant cost in grating quality andprecision.

[0012] To date, no apparatus or process for the large scale manufactureof optical fiber Bragg gratings has been identified which providesproduction speed and efficiency, ensures grating quality, and maintainsoptical fiber strength following grating formation.

SUMMARY OF THE INVENTION

[0013] The present invention is a stepwise continuous process for themanufacture of optical fiber Bragg gratings which provides the speed ofdraw tower production processes as well as the control over gratingquality available from small scale batch processes. The process of thepresent invention decouples the optical fiber draw process from thegrating writing process and provides an efficient and economicaloff-line method for producing Bragg gratings in coated optical fibers.

[0014] In a presently preferred embodiment, the present invention is aprocess for manufacturing an optical fiber Bragg grating, whichcomprises the following steps:

[0015] (a) removing at least a portion of a removable coating in atleast one predetermined section of the element to sufficiently expose anoptical fiber in the section for subsequent treatment with a source ofoptical radiation;

[0016] (b) fixing the at least one section with respect to the source ofoptical radiation;

[0017] (c) directing optical radiation from the source into the at leastone section to produce a Bragg grating therein; and

[0018] (d) covering the at least one section.

[0019] In step (a), a glass optical fiber element of any diameter orshape may be provided for processing. The optical fiber element ispreferably coated with a removable permanent or semi-permanent coatingto protect it from the environment. If necessary, the coating is removedfrom a single predetermined section of an optical fiber, from multiplesections, or from its entire length by at least one of mechanical,chemical, or thermal coating removal techniques. Complete removal of thecoating in the section is preferred, but it is only necessary tosufficiently remove the coating such that the optical radiation mayaccess the optical fiber core in the section to form the Bragg gratingtherein.

[0020] The present invention provides a process for preparing an opticalfiber for the making of an optical fiber device. In the process of theinvention, an optical fiber element is initially provided which includesan optical fiber, preferably made of a silica-based glass, which iscoated with one or more thermally removable coatings. The optical fiberhas a known initial strength, which may be evaluated by a measurement ofits median fracture stress according to ANSI/EIA/TIA 455-18B-1991(FOTP)-28). All or a portion of the coatings are thermally removed tosufficiently expose the optical fiber to allow subsequent processinginto an optical fiber device. The thermal removal is performed such thatthe optical fiber retains a percentage of its initial median fracturestress as required for its intended end use application, as measure byFOTP-28.

[0021] In addition, it is highly desirable that the thermal removal beperformed such that the homogeneity of the fiber strength is retained.The strength distribution of the fiber following thermal removal of thecoating should be narrow, as evidenced by a sufficiently high Weibullmodulus or slope, m, for an intended application as measure by FOTP-28.

[0022] Following the subsequent processing step, the optical fiberdevice may be recoated with the thermally removable coating, recoatedwith a conventional coating, or incorporated into a subsequentprocessing step.

[0023] During the coating removal step, the thermally removablecoating(s) used in the process of the invention rapidly thermallydegrade such that the optical fiber absorbs a minimum of thermal energy.In addition, the thermally removable coating(s) are sufficiently removedsuch that substantially no residue remains on the surface of the opticalfiber to interfere with subsequent processing or degrade the physicalstrength of the optical fiber. Preferably, the removable coating is apolymeric material which, with the application of heat, rapidlyde-polymerizes to lower molecular weight species which volatilize in theprocessing environment leaving little to no residue such that theoptical fiber retains an amount of its initial average fracture stress.

[0024] The thermal removal step may be performed in any suitable manner,but it is important in the process of the invention that the temperatureof the optical fiber remain sufficiently low to preserve its physicalintegrity. Thus, it is preferred that thermal removal of the coating(s)be performed with a heated gaseous stream to facilitate rapid coatingremoval and minimize overheating of the optical fiber.

[0025] The present invention also extends to optical fiber devices madeby the above-described process.

[0026] The process of the invention provides a continuous or stepwisecontinuous method for preferentially or completely removing theprotective coating(s) from a glass optical fiber with minimum physicaldamage to the glass. The fiber may then be further processed into anyone of a wide variety of optical fiber devices. The process of theinvention eliminates the potential fiber damage caused from the bladesof a mechanical stripping tool or chemicals used to swell the coating toassist in the removal processes. The process of the invention leavessubstantially no residue on the fiber surface, so that deterioration inphysical strength caused by wiping the surface of the fiber to removecoating debris does not occur. In addition, the process of the presentinvention eliminates the need to soak the fiber in flammable, corrosiveand potentially toxic solvents as routinely performed in the art, whichsimplifies the optical fiber device manufacturing process. The processof the present invention therefore dramatically reduces the exposuretime of bare optical fiber and eliminates associated handling practicesutilized in present manufacturing processes.

[0027] In step (b), the predetermined section of the optical fiber wherethe Bragg grating is to be formed is immobilized, e.g. fixed withrespect to a source of optical radiation, so that the grating writingprocess may proceed in that section with the desired degree ofprecision. The section to be processed may be immobilized in any knownmanner, but gripping the fiber adjacent a first end of the section andadjacent a second end of the section with a mechanical clamping deviceis preferred. If desired, the clamping devices may be used to applylongitudinal stress to the fiber during grating writing (step(c)) tofine-tune the wavelength of the Bragg grating formed in the processedoptical fiber section and continuously monitor its quality, or mayinclude optional means for rotationally orienting the fiber prior to orduring grating formation.

[0028] In step (c), optical radiation, preferably emitted from acoherent source such as laser, is directed into the section of theoptical fiber to be processed. A single Bragg grating or multiple Bragggratings may then be formed in the core of the predetermined section ofthe optical fiber using phase mask projection, holography, or acombination thereof. The fiber may optionally be annealed following step(c) to improve the stability of the Bragg grating.

[0029] In step (d), at least the processed section of the optical fiberwhich contains the Bragg grating, or the entire optical fiber element,is covered as required for its end use application. The covering mayvary widely depending on the intended application, and may includeapplication of temporary or permanent sleeves, or application ofmechanical devices such as connectors. However, the processed sectioncontaining the Bragg grating, or the entire optical element, istypically recoated with a protective coating to protect the opticalfiber and Bragg grating in the processed section from the environment,and to preserve the strength of the optical fiber element. Theapplication of this coating also allows an opportunity to identify thelocation of the Bragg grating formed. For example, to identify thelocation of the Bragg grating along a length of optical fiber, theprocessed section may be recoated with a coating having a differentcolor than the coating on the non-processed portion of the fiber. In thealternative, the processed section may be recoated with a clear coating,or an identifying mark, such as, for example, a bar code, may beapplied.

[0030] In another embodiment, a coating may be applied to the opticalfiber element which is sufficiently transparent to the optical radiationat the grating writing wavelength such that no coating removal step isrequired. In this embodiment, the process of the present inventioncomprises the following steps:

[0031] (a) fixing at least one section of the element with respect to asource of optical radiation; and

[0032] (b) directing optical radiation from the source through thecoating to produce at least one Bragg grating in the at least onesection.

[0033] If necessary, the optical fiber may then be further processed toprotect its optical and physical properties.

[0034] The present invention also extends to an apparatus for carryingout the process steps described above. The apparatus of the presentinvention will typically be provided as a process line with a coatingremoval station, a fiber immobilization and Bragg grating writingstation, and a packaging station. The apparatus or the stations thereofmay optionally be supplied in modular form.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035]FIG. 1 is a schematic diagram of an apparatus for continuous orstepwise continuous processing of optical fiber elements according tothe process of the present invention;

[0036]FIG. 2A is a spectrum analyzer plot of reflection vs. wavelengthfor a Bragg grating array produced according to the process of thepresent invention; and

[0037]FIG. 2B is a spectrum analyzer plot of reflection vs. wavelengthfor the Bragg grating array of FIG. 2A with one grating in the arrayplaced under longitudinal strain.

[0038]FIG. 3 is an isothermal thermogravimetric analysis (TGA) plot at400° C. of the coatings used in Example 1;

[0039]FIG. 4 is an isothermal TGA plot at 500° C. of the coatings usedin Example 1;

[0040]FIG. 5 is an isothermal TGA plot at 600° C. of the coatings usedin Example 1;

[0041]FIG. 6 is a schematic diagram of an apparatus which may be used tothermally remove the thermally removable coatings in the process of thepresent invention;

[0042]FIG. 7 is a Weibull plot showing the initial fracture strengthdistribution of optical fibers of Example 2, and the fracture strengthdistribution of the fibers following thermal removal of the coatings;

[0043]FIG. 8 is a schematic diagram of a system for continuous orstepwise continuous processing of optical fiber elements according tothe present invention; and

[0044]FIG. 9 is a schematic diagram of an apparatus which maybe used toprepare an optical fiber current sensor according to the process of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0045] The process of the present invention is typically used tomanufacture Bragg gratings from plastic or glass, preferablysilica-based glass, bare optical fibers which have been drawn and coatedon a draw tower with at least one protective coating to form an opticalfiber element. As is well known in the art, optical fiber elementsgenerally consist of a bare fiber(s), and one or more buffer layersaround the bare fiber to protect the optical fiber from microbendinglosses and abrasion (see, for example, Sterling, Technician's Guide toFiber Optics, (1993), at page 73). In the present application, the term“bare fiber” or “optical fiber” refers to a section of the fiber opticelement from which the buffer and external strength members have beenremoved. If a non-strippable protective layer lies beneath the buffer,the protective layer is considered part of the bare fiber.

[0046] Referring to an apparatus 10 schematically illustrated in FIG. 1,the optical fiber elements 12 which may be incorporated into the processof the present invention are typically stored wound on spools. Theoptical fiber elements 12 used in the apparatus and process of thepresent invention are provided with at least one coating. In the processof the present invention, the coatings are applied to a bare opticalfiber, or a bare optical fiber with a non-strippable protective layer,and do not include the strength members or the jacket which make up theouter layers of an optical fiber cable.

[0047] As is well known in the art, the coatings which are applied tothe bare fiber at the draw tower will vary widely depending on thefiber's intended end use application. The coatings are typicallyselected to protect the delicate optical fiber structure from mechanicaland/or environmental damage. Optionally, a coating may be applied to thebare fiber which is also sufficiently transparent to a particularwavelength of optical radiation such that the selected wavelength may bedirected through the coating to form a Bragg grating. Such a coatingwill be referred to herein as a transparent coating. However, otherdesign considerations may require that a coating be selected which isnot transparent to the wavelength of optical radiation used to form theBragg grating in the optical fiber core. If the coating material on theoptical fiber element 12 is not substantially transparent to thewavelength of optical radiation used to write the Bragg grating, it isnecessary that the coating be sufficiently removed to allow the selectedwavelength to access the optical fiber core. The non-transparentcoatings may comprise any material substantially completely removable byat least one of a mechanical, chemical or thermal technique. Suchcoatings will be referred to herein as removable coatings.

[0048] In the apparatus and process of the present invention, theoptical fiber element 12, or a specified length thereof, is unwound froma tension controlled payoff spool 14. The payoff spool 14 may include anoptional rotational optical coupler (not shown) to provide real-timeinformation regarding the Bragg grating writing process. The opticalelement 12 is pulled through a series of alignment pulleys 16 by a drivecapstan apparatus 18 to an optional coating removal station 20. Ofcourse, if a coating is applied to the optical fiber element 12 which issubstantially transparent to the wavelength of the optical radiation tobe used to write the Bragg grating in the optical fiber core, no coatingremoval step is required in the process of the present invention.However, the apparatus and process of the present invention will befurther described below assuming that a removable coating is applied tothe optical fiber which is not transparent to the wavelength of theoptical radiation to be used to write the Bragg grating in the opticalfiber.

[0049] In the coating removal station 20 it is necessary that asufficient portion of the coating be removed from a predeterminedsection of the optical fiber element such that optical radiation mayaccess the optical fiber core to form a Bragg grating therein. Thepredetermined section of the optical fiber may comprise the entirelength of the optical fiber element, or any length less than the entirelength.

[0050] As noted above, the removable coating may be substantiallyremoved from the optical fiber in the coating station 20 by any ofmechanical, chemical, or thermal means, or combinations thereof.Regardless which method of coating removal is employed in the coatingremoval station 20, it is important to select a removable coating foruse in the apparatus and process of the present invention that may besubstantially completely removed from the optical fiber to permitsubsequent processing with optical radiation to form a Bragg grating inthe optical fiber core. In the coating removal station 20, the removablecoating is preferably completely removed, but partial removal of thecoating may also be used if such removal is sufficient to permitsubsequent processing. However, any residue which remains in contactwith the bare fiber surface following incomplete removal of a coating,particularly carbonaceous residue following incomplete thermal removal,creates a local stress concentration, which may significantly degradethe tensile strength of the optical fiber. In addition, the small flakeswhich remain on the surface of the optical fiber following incompletecoating removal may block optical radiation and interfere withsubsequent processing steps.

[0051] For the purposes of the present application, the termsubstantially complete removal applies to any polymeric coating which,following removal, has a residuum of less than about 10% by weight,preferably less than about 5% by weight, based on the initial coatingweight. Coatings which may be used in the process of the invention maybe identified with a wide variety of analytical techniques, such asthermogravimetric analysis (TGA).

[0052] To be useful in the process of the present invention, which ispreferably continuous or stepwise continuous, the substantially completeremoval of the coating should be completed in a commercially feasibletime, which for the purposes of the present application, is less thanabout 15 seconds, preferably less than about 10 seconds, and mostpreferably less than about 1 second. The thickness of the removablecoating or coatings used in the process of the present invention mayvary widely depending on the intended application, but a conventionalcoating thickness of about 15μ to about 35μ is normally used.

[0053] The substantially complete coating removal referred to above ispreferably accomplished such that fiber tensile strength is not reducedbelow a predetermined level required for a particular end useapplication. For the purposes of the present invention, the tensilestrength of the optical fiber is measured by evaluation of the medianfracture stress according to ANSI/EIA/TIA-455-28B-1991, which will bereferred to herein as FOTP-28. Preferably, in the process of the presentinvention the median fracture stress of the optical fiber followingcoating removal should not be reduced more than about 50%, based on theinitial median fracture stress of the optical fiber prior to coatingremoval.

[0054] However, optical fiber strength testing is statistical in nature,and many individual fibers, each of which is representative of a givenpopulation, must be tested for strength. The result is reported for thepopulation as a whole as a strength distribution, and this distributionis characterized by slope, m, (also referred to as the Weibull modulus)of the well-known Weibull plot. In the present process, the Weibullmodulus is a measure of the homogeneity of fiber strength retainedfollowing coating removal. The strength distribution of the fiberfollowing coating removal is preferably narrow, as evidenced by asufficiently high Weibull modulus or slope, m, for an intendedapplication as measured by FOTP-28.

[0055] A large m in excess of about 100 corresponds to a uniform ornarrow strength distribution, and suggests that a characteristicfracture stress exists for the fiber and that the probability of failuredoes not become significant until the applied tensile stress approachesthat characteristic value. On the other hand, a low Weibull modulus ofless than about 20 suggests that the probability of failure issignificant at almost any applied stress, and is indicative of lowmechanical reliability. Preferably, the thermal removal of the coatingmust reduce the initial (e.g., prior to thermal coating removal) Weibullmodulus not more than about 50%.

[0056] In the coating removal station 20, the removable coating may beremoved by any conventional means from the entire optical fiber or froma section of length. For example, the removable coating may bemechanically “stripped” from the bare fiber with a knife or similartool. The removable coating may be chemically removed by soaking in anappropriate solution. Many different chemical solutions may be used, andconcentrated sulfuric acid or a combination of sulfuric acid andhydrogen peroxide are typical examples. In the alternative, acombination of chemical and mechanical coating removal techniques may beused. For example, the removable coating may be soaked in a solvent,such as acetone, to swell the coating, and then the swelled coating maybe mechanically stripped from the fiber. In the alternative, heat may beapplied to the optical fiber by any conventional means to deteriorate orburn away the coating.

[0057] However, mechanical stripping with a knife or tool may causescratches on the glass fiber surface, which ultimately lead to finecracks and decreased fiber strength. Chemical stripping techniques oftenleave a residue on the fiber surface which reduces fiber strength andinterferes with subsequent processing steps. If heat is applied toremove the coating, the charred residue which results reduces fiberstrength and may require additional coating removal steps prior toprocessing. In addition, the optical fiber absorbs heat during coatingpyrolysis, which may result in fiber embrittlement. While any coatingremoval procedure may be employed in the apparatus and process of thepresent invention, thermal removal is presently preferred because it isbelieved to have the least deleterious effect on the strength of theoptical fiber. See co-pending U.S. application Ser. No. 08/631,491,which is incorporated herein by reference. The removable coatings whichare preferred for use in the apparatus and process of the presentinvention are described in co-pending U.S. application Ser. No.08/631,491, which, as stated above, is incorporated herein by reference.

[0058] As is well known in the art, fiber optic cables generally consistof optical fiber(s), and one or more buffer layers around the fiber toprotect the optical fiber from microbending losses and abrasion (see,for example, Sterling, Technician's Guide to Fiber Optics, (1993), atpage 73). In the present application, the term “bare fiber” or “opticalfiber” refers to the portion of the fiber optic cable from which thebuffer and external strength members have been removed. If anon-strippable protective layer lies beneath the buffer, the protectivelayer is considered part of the bare fiber. In this application, theterm “removable coating” refers to any coating layer applied to theoptical fiber, bare fiber, or bare fiber with a non-strippableprotective layer, but does not include the strength members or thejacket which make up the outer layers of the cable.

[0059] The optical fiber which is used in the process of the presentinvention may be made of any material, such as plastic or glass.Conventional silica-based glass materials are preferred.

[0060] The removable coating(s) used in the process of the presentinvention may be any polymeric material which may be easily coated onthe optical fiber with conventional equipment. The removable coating(s)must be subsequently thermally removed to leave substantially no residueon the surface of the optical fiber. In addition, thermal removal mustoccur in a commercially feasible time such that the fiber does notabsorb sufficient heat to reduce its tensile strength below a level.

[0061] First, in selecting a polymeric coating material for use in theprocess of the invention, it is important that the material besubstantially completely removable in a commercially feasible time fromthe optical fiber. Carbonaceous residue which remains in contact withthe bare fiber surface following incomplete thermal removal of a coatingcreates a local stress concentration, which degrades the tensilestrength of the fiber. In addition, the small flakes which remain on thesurface of the optical fiber following incomplete coating removal mayinterfere with subsequent processing steps.

[0062] For the purposes of the present application, the termsubstantially complete removal refers to any polymeric coating which,following thermal removal, has a residuum of less than about 10% byweight, preferably less than about 5% by weight, based on the initialcoating weight, after thermal treatment in air at about 300° C. to about900° C., preferably about 400° C. to about 700° C., most preferablyabout 500° C. to about 600° C. Coatings which maybe used in the processof the invention may be identified with a wide variety of analyticaltechniques, such as thermogravimetric analysis (TGA).

[0063] Of course, to be useful in the process of the present invention,which is preferably continuous or stepwise continuous, the substantiallycomplete removal of the coating is preferably completed in acommercially feasible time, which for the purposes of the presentapplication, is less than about 15 seconds, preferably less than about10 seconds, and most preferably less than about 1 second. The thicknessof the removable coating or coatings used in the process of the presentinvention may vary widely depending on the intended application, but aconventional coating thickness of at 15μ to about 35μ is normally used.

[0064] Second, the substantially complete thermal removal referred toabove preferably is accomplished such that the fiber does not absorbsufficient heat to reduce its tensile strength below a level requiredfor a particular end use application. For the purposes of the presentinvention, the tensile strength of the optical fiber is measured byevaluation of the median fracture stress according toANSI/EIA/TIA-455-28B-1991, which will be referred to herein as FOTP-028.In this test procedure, the optical fiber under test is threaded betweena gripping mechanism and a capstan. The fiber is then elongated at afixed strain rate until it breaks. The rat of elongation is express as%/min., relative to the gauge length, and tensile load at failure ismeasured by an appropriate load cell. The fracture stress, σ_(f), is theprimary parameter used to support strength performance, and iscalculated as follows:

σ_(f) =T/A _(g)

[0065] where T is the force (tension) experienced by the specimen atfailure, and A_(g) is the cross-sectional area of the fiber. See, forexample, Tariyal et al., Ensuring the Mechanical Reliability ofLightguide Fiber, Western Electric Engineer, Winter 1980.

[0066] For the purposes of the present application, the strength of anoptical fiber is expressed as a median fracture stress for a particularpopulation, and this median fracture stress value must remain above alevel following thermal coating removal. Typically, the thermal removalof the coating must reduce the initial median fracture stress, not morethan about 50%, preferably not more than about 25%, and most preferablynot more than about 15%.

[0067] However, optical fiber strength testing is statistical in nature,and many individual fibers, each of which is representative of a givenpopulation, must be tested for strength. The result is reported for thepopulation as a whole as a strength distribution, and this distributionis characterized by slope, m, (also referred to as the Weibull modulus)of the well-known Weibull plot. See, for example, Bittence, SpecifyingMaterials Statistically, Machine Design, vol. 50, No. 2 (1978); Epstein,Statistical Aspects of Fracture Problems, Journal of Applied Physics,vol. 19, February 1948; Bacon, Silica Optical Fibers Application Note,3M, June 1995. In the present process, the Weibull modulus is a measureof the homogeneity of fiber strength retained following thermal coatingremoval. The strength distribution of the fiber following thermalcoating removal should be narrow, as evidenced by a sufficiently highWeibull modulus or slope, m, for an intended application as measured byFOTP-28.

[0068] A large m in excess of about 100 corresponds to a uniform ornarrow strength distribution, and suggests that a characteristicfracture stress exists for the fiber and that the probability of failuredoes not become significant until the applied tensile stress approachesthat characteristic value. On the other hand, a low Weibull modulus ofless than about 20 suggests that the probability of failure issignificant at almost any applied stress, and is indicative of lowmechanical reliability. Typically, the thermal removal of the coatingmust reduce the initial (e.g., prior to thermal coating removal)Weilbull modulus not more than about 50%, preferably not more than about25%, and most preferably not more than about 15%.

[0069] The optical fiber fracture stress following coating removal issensitive to the amount of heat applied to the fiber. Therefore, it isimportant in the process of the invention that heat be applied duringthe coating removal step such that a minimum of thermal energy istransferred to the optical fiber. The heat may be applied to thermallyremove the coating in any appropriate manner which preserves thephysical integrity of the optical fiber, such as with a heatedresistance filament or other radiant type heat source, a CO₂ laser or aheated gaseous stream. FIG. 6 is a schematic representation of thecoating removal step of the process of the present invention, in whichsections of coating may be preferentially removed from the optical fiberin a controlled fashion. In FIG. 6, an optical fiber element 12, whichis coated with a thermally removable coating 13, is heated by alocalized heat source, such as a source of heated gas 38, thus revealinga portion of a residue free glass surface 15.

[0070] Thermal removal of the coating is preferably performed using aheated gaseous stream. While not wishing to be bound by any theory, theheated gaseous stream is believed to assist in volatilization of thepolymeric coating material and sweep away the de-polymerized productwith minimum transfer of heat to the optical fiber. The gaseous streammay comprise any gas or mixture of gases, including air, nitrogen,argon, and the like, and nitrogen is preferred for its inert propertiesand availability. Gas mixtures containing oxygen are less desirable foruse in the process of the present invention, since the heat ofcombustion generated during thermo-oxidative processes increases thetemperature of the glass and degrades its strength characteristics.

[0071] The gaseous stream may be applied by any suitable technique, suchas with an air gun or an air knife. However, an air knife is preferredfor a commercially feasible continuous or stepwise continuous processfor preferential removal of a predetermined length of coating. Thetensile strength of the fiber may be optimized by using a heat source,fixed at a desired distance, at the appropriate temperature to removethe thermally removable coating. Of course, the parameters will varywidely depending on the coating selected, coating thickness, theprocessing time, gas flow rate, and gas temperature. For example, aresistance wire coiled in a circular tube with a restricted outputplaced about 2 to about 10 mm from the surface of the coating,preferably about 5 mm, with a gas flow rate of about 1 to about 3 scfm,and a gas stream temperature from about 400° C. to about 900° C.,preferably about 600° C. to about 700° C., has been found effective forsuitable thermal removal of the coating described in this invention.

[0072] A schematic diagram of a continuous or stepwise continuousprocess for optical sensor manufacture according to the presentinvention is illustrated in FIGS. 1 and 8. In FIG. 8, the coated opticalfiber element 110 is unwound from a tension controlled payoff spool 112through a precision positioning device 114. The fiber element 110 thenenters a fiber heater 138, which may comprise, for example, an airknife, for thermal removal of a length of the removable coating. Thebare fiber 118 which is exposed by coating removal then proceeds into aprocessing zone 120, where the fiber is treated to form an opticalsensor using processes known in the art. Following this processing step,the optical fiber sensor (not shown) may be further processed or mayenter an optional in-line coater 122 to protectively re-coat the portionof bare fiber now containing the sensor. The recoated optical fibersensor would then enter an optional coating curing zone 124. The sensorwith cured coating is then drawn onto an isolation capstan 126 andfinally wound onto a tension controlled take-up spool 128. The recoatingprocess is recommended prior to subjecting the bare fiber to anymechanical detriment, such as an isolation capstan of fiber guide, asthis severely degrades the strength characteristics of the fiber.

[0073] Any number of treatment steps may be performed on the exposedsection of bare optical fiber 118 in the processing zone 120 of FIG. 6to produce a broad array of optical fiber devices for variousapplications. For example, the exposed section of the bare optical fiber118 may be treated with a laser to form an optical fiber Bragg grating.The Bragg grating may be produced in the optical fiber by any methodknown in the art, such as, for example, phase mask projection orholography. See, for example, Farries et al., Fabrication andPerformance of Packaged Fiber Gratings for Telecommunications, and Rizviet al., Production of Bragg Gratings in Optical Fibers by Holographicand Mask Projection Methods, Institution of Electrical Engineers,Optical Fiber Gratings and Their Applications, January 1995. Theresulting optical fiber Bragg grating reflects an extremely narrowspectral band of an incoming signal, and may be used in applicationssuch as fixed and tunable filters, fiber and diode lasers, wavelengthdivision multiplexing, fiber amplifiers and sensors. See, for example,Measures et al., Grating Fiber Optic Sensing for Bridges and OtherStructures, 2d European Conference on Smart Structures and Materials,October 1994; Melle et al., Practical Fiber-Optic Bragg Grating StrainGauge System, Applied Optics, vol. 32, no. 19, July 1993; and Alavie etal., A Multiplexed Bragg Grating Fiber Laser Sensor System, IEEEPhotonics Technology Letters, vol. 5, No. 9, September 1993.

[0074] Other types of optical fiber devices which may be produced in theexposed bare portion 118 of the optical fiber include, for example,current sensors. Presently, as described in co-pending U.S. applicationSer. No. 08/205,880 to Cronk et al., and U.S. Pat. No. 5,051,577 to Lutzet al., the disclosures of which are hereby incorporated by reference,useful optical fiber sensor coils (birefringence<3°., with immeasurablechange from −40 to +80° C.) are produced by removing the coating from anappropriate length of optical fiber by mechanical stripping. Thestripped optical fiber is cleaned with alcohol to provide a cleansurface for cleaving and fusion splicing processes. The fiber is thenplaced in an annealing fixture which is placed into an annealing oven.The annealing process is performed and the mold is removed from theoven.

[0075] In sensor manufacture, mechanical stripping and subsequenthandling of the bare sensor fiber exposed the fiber to potentialmechanical damage. Such damage and subsequent strength reduction of thiscritical portion of the optical sensor could cause the fiber to breakand the sensor to fail in the field. In some processes the fiber coatingis softened by soaking in solvent to render the coating more easilystripped by mechanical means. The fiber can then be further processed.The process of the invention eliminates the hazard of placing opticalfibers which may still contain traces of flammable solvents directlyinto an annealing furnace. Likewise the use of the process of thepresent invention eliminates the hazard associated with solventpre-soaking the coating and the strength degradation associated withmechanical stripping in these processes.

[0076]FIG. 9 illustrates an embodiment of a fiber coil holder 200 formaking an optical fiber current sensor using the thermal coating removalprocess of the present invention. A circular groove 232 is formedintegrally with a plate 230 by sandblasting into the surface. Channels234 may also be sandblasted into the surface to provide guides for theterminal ends of the fiber coil. Holder 230 is used as a mold or formfor annealing by loosely winding the coated optical fiber element 210 inthe circular groove 232 and then placing the holder 230 in a heatedenvironment to substantially thermally remove the coating.

[0077] In addition to the specific applications mentioned above,following thermal removal of a predetermined portion of the removablecoating, the processing step of the process of the invention may be usedto prepare optical fiber splitters and couplers. Further, if the opticalfiber element is embedded in a composite material, such asgraphite/epoxy composite, a thermally removable coating may be used toprotect the optical fiber during composite preparation, and the coatingmay be subsequently removed via de-polymerization and diffusion duringthe thermal processing step which cures the composite. A thermallyremovable coating may also be used as a carrier of a liquid component,which is released upon removal of the coating during the processing stepallowing the liquid to wet the fiber or cure the surrounding compositematerial. Additionally, the thermally removable coating permitsseparation of the interdependency of the draw process with furtherprocess steps which are not compatible with standard draw techniques,such as hermetic coating applications.

[0078] Following the processing steps, the optical fiber devicesproduced may optionally be recoated with a protective coating. Theprotective coating may be the same as the thermally removable coating,or may be selected from any coating material known in the art. Followingthe recoating step, if the thermally removable coating is used, theprotective coating may be treated to make it less thermally susceptibleto removal if necessary to provide improved thermal or chemicalresistance to the completed optical fiber device. Following therecoating step, the completed optical fiber sensor may be wound on atake-up reel or further processed as necessary for its intendedapplication.

[0079] The invention will be further described with reference to thefollowing non-limiting examples.

EXAMPLES Example 1

[0080] Coatings which may be used in the process of the invention may beidentified with a wide variety of analytical techniques, such asthermogravimetric analysis (TGA). FIG. 1 is an isothermal plot at 400°C. in air of 10.5 mg samples of three cured acrylate films, availablefrom DSM Desotech, Inc., Elgin, Ill., which are known to be useful asoptical fiber coatings. About 66% by weight, based on the initial amountof coating material under test, of Example coating D, an acrylated epoxyavailable from DSM Desotech under the product identification DSM3471-2-137, volatilizes after three hours at 400° C., compared to about79% by weight of Example coating C, an acrylated urethane available fromDSM Desotech under the product identification DSM 3471-2-113. However,about 95% by weight of Example coating B, a multi-functional acrylateavailable from DSM Desotech under the product identification DSM 5000-2,volatilizes at 400° C. over the same time period.

[0081]FIG. 2 is an isothermal plot of coatings B, C and D at atemperature of 500° C. in air. Coating B, DSM 5000-2, clearly leavesless residue following a 30 minute heating period than coatings C and D.Similar results are achieved at 600° C., as shown in FIG. 3. Thus, it isclear that coating B, DSM 5000-2, which has a residuum of less thanabout 5% by weight, based on the initial coating weight under test,following about 5 minutes of exposure at a temperature of about 500-600°C., is substantially completely removed in a commercially feasible timeperiod, and is a suitable coating for the process of the presentinvention.

Example 2

[0082] To determine the effect of the thermal removal of the coatings ofExample 1 on the strength characteristics of an optical fiberpopulation, three fiber draws were performed to prepare optical fiberelements coated with each of the three Example coatings B, C, and Dabove to yield optical fibers B, C, and D respectively. The examplecoatings were coated using conventional pressure coating techniques ontoa fiber freshly drawn from a fire polished high purity silica preform ata draw speed of 55 meters per minute on a standard production fiber drawtower. The diameter of the silica fiber was 80 μm and the final coatedfiber diameter was 128 μm.

[0083] A fracture strength test was performed on each optical fiber B, Cand D by the dynamic fatigue method of FOTP-28 at 9%/minute strain rate,4 meter gauge length, using ten specimens per test, for each of thefibers drawn to establish a baseline strength distribution. For theanalysis of the fiber strength distribution of fiber after thermalremoval of the coating, the specimen was threaded between the mechanicalgripping sensor and the capstan of the dynamic fatigue equipment priorto thermal removal of the coating. The coating was removed by the methoddescribed and the strength analysis conducted after briefly allowing thefiber to cool.

[0084] A hot air gun, Model 27046, manufactured by Dayton ElectricManufacturing Company of Chicago, Ill., was used to remove 3-4 inchlengths of coating from optical fiber B. The hot air gun used was ratedat 20 amperes, had a rated operating temperature of 1,000° F. (500-600°C.), and was hand-held at an approximate distance of 2.5 inches (6.5 cm)from the optical fiber.

[0085] The strength population of optical fiber B had an initial (priorto coating removal) baseline median fracture stress of about 650 KPSI.Following substantially complete coating removal over a section with thehot air gun, the fracture stress of the B-coated optical fiberpopulation dropped to about 550 KPSI, a reduction of approximately 15%.

[0086] The Dayton hot air gun was used to remove the coatings fromoptical fibers C and D. However, no combination of coating removalconditions allowed complete removal of the coating to provide a cleanfiber surface.

[0087] An air knife, available from Air Knife Inc. of Charlotte, N.C.,which provides a more concentrated gaseous stream at a highertemperature than the Dayton hot air gun, was then utilized to thermallyremove the coating from optical fibers C and D. The air knife usedconsisted of a stainless steel tube with a resistance wire coiled insideto heat a gas stream. The tube was necked down to an inside diameter ofabout 3 mm at the downstream end to concentrate the gas flow exiting thetube. A nitrogen flow of 1.5 to 2.3 scfm was input to the supply end ofthe tube and the temperature of the exiting gas stream was measured asabout 600° C. to about 700° C. The heated gas stream was applied at adistance of about 3 mm to about 5 mm from the coated fiber surface.

[0088] Optical fiber C, coated with acrylated urethane (DSM 3471-2-113),gave an initial median fracture strength of about 700 KPSI and respondedwell to thermal removal of the coating. However, the median strength offiber C was degraded to 160 KPSI during the coating removal process.This is a reduction in strength of 78%.

[0089] Optical fiber D, coated with acrylated epoxy (DSM 3471-2-137),gave an initial median fracture strength of about 700 KPSI. Theapplication of heat with the air knife created a charred residue whichremained on the fiber regardless of the temperature and flow rate of theheated gas stream applied by the air knife. This condition results in afiber unsuitable for the further processing necessary to generate anoptical fiber device using the process of the present invention.Therefore, post removal dynamic fatigue analysis was not conducted onoptical fiber D.

[0090]FIG. 7 is a Weibull plot summarizing the strength population ofthe optical fibers before and after removal of the coatings B, C and Dof Examples 1-2. There was a substantial reduction in the medianstrength of optical fiber C upon thermal removal of the coating and thecoating cannot be effectively removed from optical fiber D. However,removal of coating B resulted in excellent strength retention for fiberB, so coating B would clearly be preferred for use in the process of thepresent invention.

[0091] Following thermal removal in the commercially feasible timereferred to above, the preferred removable coatings will have a residuumof less than about 10% by weight, preferably less than about 5% byweight, based on the initial coating weight, after thermal treatment inair at about 300° C. to about 900° C., preferably about 400° C. to about700° C., most preferably about 500° C. to about 600° C. The thermalremoval of the preferred removable coating should preferably reduce theinitial median fracture stress not more than about 50%, preferably notmore than about 25%, and most preferably not more than about 15%. Thepreferred removable coatings used in the process of the presentinvention should reduce the initial (e.g., prior to coating removal)Weibull modulus of the optical fiber not more than about 50%, preferablynot more than about 25%, and most preferably not more than about 15%.

[0092] The optical fiber fracture stress following removable coatingremoval is sensitive to the amount of heat applied to the fiber.Therefore, it is important in the process of the invention that heat beapplied in the coating removal station 20 such that a minimum of thermalenergy is transferred to the optical fiber. The heat may be applied tothermally remove the removable coating in any appropriate manner whichpreserves sufficient optical fiber strength for a particular end useapplication, such as, for example, with a heated resistance filament orother radiant type heat source, a CO₂ laser or a heated gaseous stream.Thermal removal of the removable coating in the coating removal station20 is preferably performed using a heated gaseous stream. While notwishing to be bound by any theory, the heated gaseous stream is believedto de-polymerize the removable coating material and sweep away thevolatilized product with minimum transfer of heat to the optical fiber.The gaseous stream may comprise any gas or mixture of gases, includingair, nitrogen, argon, and the like, and nitrogen is preferred for itsinert properties and availability. Gas mixtures containing oxygen areless desirable for use in the process of the present invention, sincethe heat of combustion generated during thermo-oxidative processesincreases the temperature of the optical fiber glass and degrades itsstrength characteristics.

[0093] The gaseous stream may be applied by any suitable technique, suchas with an air gun or an air knife. However, an air knife is preferredfor a commercially feasible continuous or stepwise continuous processfor preferential removal of a length of removable coating from a sectionof optical fiber. The tensile strength of the fiber following removablecoating removal may be optimized by using a heat source, fixed at adesired distance, at the appropriate temperature to remove the removablecoating. Of course, the parameters will vary widely depending on thecoating selected, coating thickness, the processing time, gas flow rate,and gas temperature. For example, a resistance wire coiled in a circulartube with a restricted output placed about 2 to about 10 mm from thesurface of the coating, preferably about 5 mm, with a gas flow rate ofabout 1 to about 3 scfm, and a gas stream temperature from about 400° C.to about 900° C., preferably about 600° C. to about 700° C., has beenfound effective for suitable thermal removal of the removable coating.

[0094] Following the coating removal step, a section 22 of the opticalfiber from which the coating has been substantially completely removedenters a fiber immobilization and grating writing station 24. Thestation 24 includes means for fixing the section 22 of the optical fiberwith respect to a source of optical radiation, and means for applyingthe optical radiation to the section 22 to form a Bragg grating at awavelength or Bragg condition in the optical fiber core of the section22.

[0095] Any means for immobilizing the section 22 may be used which holdsthe optical fiber element 12 sufficiently stationary such that a Bragggrating may be written in the core of the optical fiber in the section22 with a desired Bragg condition. A presently preferred means forimmobilizing the section 22 to be processed comprises a first tensioningclamp 26 and a stage clamp 28. The clamps 26 and 28 mechanically grip afirst end and a second end, respectively, of the section 22. To preventdamage to the optical fiber, it is preferred that the clamps 26 and 28engage coated portions of the optical fiber element adjacent the barefiber in the section 22.

[0096] The clamps 26 and 28 may simply hold the section 22 taut andfirmly in place while the grating is written in the optical fiber, ormay be used to apply a predetermined longitudinal strain to the opticalfiber section 22 to produce a predetermined Bragg condition in thesection 22. It is well known in the art that strain may be applied to anoptical fiber during the Bragg grating writing process to tune the Braggcondition of the grating, such as the resonant frequency. Typically, thestrain applied to the fiber is limited by its mechanical strength, andthe resulting elongation should not exceed about 10% of the original(pre-stretched) length of the fiber. The preferred elongation is lessthan about 7%, most preferably less than about 5%. See, for example,U.S. Pat. No. 5,384,884 to Kashyap et al.; Byron and Rourke, Fabricationof Chirped Fibre Gratings by Novel Stretch and Write Technique,Electronics Letters, vol. 1, no. 31 (January 1995); and Zhang et al.,Tuning Bragg Wavelength by Writing Gratings on Prestrained Fibers,Photonics Technology Letters, vol. 6, no. 7 (July 1994).

[0097] Application of longitudinal strain may also be used to multiplexgratings along the length of the optical fiber. Since “downstream”gratings in sections of the optical fiber which are not in tension passthe wavelength of light necessary to monitor the grating in the sectionunder tension, periodic stretching of the optical fiber may be used tomonitor in real time the accuracy of the grating writing process. SeeU.S. Pat. No. 5,384,884 to Kashyap et al.; Campbell & Kashyap, SpectralProfile and Multiplexing of Bragg Gratings in Photosensitive Fiber,Optics Letters, vol. 16, no. 12 (June 1991).

[0098] The longitudinal strain applied to the section 22 may be variedby adjusting the tensioning clamp 26. The tensioning clamp may compriseany mechanical means (not shown in FIG. 1) for applying a continuouslyvariable strain to the section 22, for example, a clamped micrometer, apiezo-electric translation stage, or a simple weight. A means forcontinuously monitoring the strain, such as, for example, a strain gaugewith closed loop control of the tensioning clamp 26, may be incorporatedin the apparatus to assist in the automation of the Bragg grating writeprocess. The stage clamp 28 will typically be a simple clamp only, butmay also be capable of applying varying levels of strain to the section22. Optionally, the tensioning clamp 26 and the stage clamp 28 mayinclude means for rotating the optical fiber section 22 about itslongitudinal axis. For example, this rotational capability may be usedto provide an appropriate Bragg grating synthesis. The rotation meansmay also be used to produce a Bragg grating having a predeterminedorientation with respect to an internal polarization axis of apolarizing or polarization-maintaining optical fiber element. Forexample, to determine the rotational orientation of a polarizing orpolarization-maintaining optical fiber with respect to some externalreference direction, the optical alignment imaging system and rotatableclamp mechanism described in U.S. Pat. No. 5,013,345 may be used.

[0099] Once the section 22 of the optical fiber to be processed has beenimmobilized, optical radiation may be applied to the section 22 in theimmobilization and grating writing station 24 to produce one or moreBragg gratings in the optical fiber core of the section 22. The Bragggrating may be produced in the optical fiber section 22 by any methodand any optical system 25 known in the art, such as, for example, byphase mask projection or holography. See, for example, Farries et al.,Fabrication and Performance of Packaged Fiber Gratings forTelecommunications, and Rizvi et al., Production of Bragg Gratings inOptical Fibers by Holographic and Mask Projection Methods, Institutionof Electrical Engineers, Optical Fiber Gratings and Their Applications,January 1995.

[0100] The presently preferred method for processing the section 22 toform a Bragg grating is shown schematically in FIG. 1. In this techniquecoherent optical radiation 27 from an excimer laser 25 a is directedthrough a phase mask 25 b and enters the core of the optical fiber inthe section 22 in a direction generally normal to the length of thesection 22. Preferably, the distance and orientation of the section 22with respect to the phase mask 25 b is precisely maintained by anarrangement of machined grooves (not shown) in a write head plate 29.

[0101] Following writing of the Bragg grating in section 22 of theoptical fiber, the processed section 22 is transported into an optionalannealing unit 30. As is well known in the art, annealing of an opticalfiber Bragg grating ensures that the optical properties of the gratingwill remain constant over an extended period of time. See Erdrogan etal., Decay of Ultraviolet-Induced Fiber Bragg Gratings, J. Appl. Phys,vol. 76, July 1994, at 73. The annealing is typically performed bypassing the optical fiber section 22 which contains the Bragg gratingthrough a heated chamber or zone (not shown). In the heated chamber,radiant or forced air heat is applied for a period of time inverselyproportional to the applied temperature to anneal the section 22 of theoptical fiber. The required annealing time will vary depending on thecharacteristics of the optical fiber element and the desired end useapplication, but, typically, a one minute dwell in the heating zone atabout 300° C. is sufficient.

[0102] Following the optional annealing step, the processed section 22is transported into a packaging unit 40. In the packaging unit 40, thesection 22 of optical fiber in which the Bragg grating is written iscovered or packaged for its particular end use application. For example,in the packaging section 40 the section 22 may be recoated with atemporary or permanent coating, temporary or permanent rigid or flexiblesleeves may be attached, or mechanical devices such as connectors may beaffixed to cover the section 22. Preferably, in the packaging section 40the processed section 22 which contains the Bragg grating, or the entireoptical fiber element 12, is recoated with any conventional temporary orpermanent protective coating.

[0103] If the original removable coating provided on the optical fiberelement 12 is in place along the entire fiber length with the exceptionof the bare section 22, to identify the location of the Bragg grating(s)along the fiber length the entire optical fiber element 12, includingthe section 22, may be re-coated with a material that is visuallydistinguishable from the original removable coating. For example, if theoriginal removable coating is colored, the recoat material applied inthe packaging section 40 may be clear, or vice-versa. In thealternative, the recoat material applied in the packaging section 40 mayhave a different thickness or texture than the original removablecoating. In addition to or instead of the recoat color identifier, theposition of the Bragg grating in the section 22 may be identified byapplying human readable or machine readable indicia to the recoatedsection 22. Examples may include a bar code, colored bars, machinereadable characters, or any combination thereof.

[0104] Following packaging, the recoated section 22 may be transportedto an optional cure unit 50 if necessary to cure the coating and/orindicia applied in the recoat unit. The cure may be performed by anyappropriate method known in the art.

[0105] Following the curing step, the completed optical fiber elementwith at least one Bragg grating written therein is routed through aseries of alignment pulleys 60 and re-wound on a take-up spool 62 forstorage or subsequent processing.

[0106] While the apparatus and process of the present invention havebeen described with respect to formation of a single optical fiber Bragggrating in a single section 22 of the optical fiber element 12, itshould be apparent to those of ordinary skill in the art that thepresent apparatus and process may also be used to form multiple Bragggratings in a single predetermined section of the optical fiber element,or, individual Bragg gratings in multiple sections of a single opticalfiber element, without splicing.

[0107] For example, if the removable coating were removed from a firstsection of an optical fiber element in the coating removal station 20 ofthe apparatus shown in FIG. 1, the first section could subsequently beadvanced to the immobilization and grating writing station 24 as shownin FIG. 1. While the first section is treated with the laser in theimmobilization and grating writing section to produce individual ormultiple Bragg gratings therein, the removable coating is removed from asecond section of the optical fiber element in the coating removalstation. When the first section is advanced to the annealing station 30,the second section may be advanced into the immobilization/writingstation 24 to produce individual or multiple gratings therein, and athird section may then be treated in the coating removal station 20, andso on.

[0108] When such a procedure is used to produce multiple gratings in asingle optical fiber, strain may be applied to the section in theimmobilization/writing station to continuously real time monitor itscharacteristics during formation. For example, FIG. 2A is a spectrumanalyzer plot of reflection vs. wavelength for a series of 15 individualgratings written on 1.2 meter centers in a single optical fiber elementto form a grating array using the process of the present invention. Thecenter wavelength of the Bragg gratings in the array (see peak A in FIG.2A) is about 1551 nm. In FIG. 2B, the fifteenth grating in the array wasplaced under longitudinal strain, and its center wavelength was observedto shift to about 1554 nm (see peak B in FIG. 2A). The center wavelengthof the fourteen non-strained gratings in the array was observed toremain constant at about 1551 nm (see peak A in FIG. 2B), whichdemonstrates the uniformity of the characteristics of the gratings inthe array.

[0109] It will be understood that the exemplary embodiments describedherein in no way limit the scope of the invention. Other modificationsof the invention will be apparent to those skilled in the art in view ofthe foregoing description. These descriptions are intended to providespecific examples of embodiments which clearly disclose the presentinvention. Accordingly, the invention is not limited to the describedembodiments or to the use of the specific elements, dimensions,materials or configurations contained therein. All alternativemodifications and variations which fall within the spirit and scope ofthe appended claims are included in the present invention. OMB No.0651-0011 INFORMATION Atty. Docket No.: Serial No.: DISCLOSURE52778USA3B.011 Unknown STATEMENT Applicant(s): James C. Novack et al.Filing Date: Group: December 21, 2000 11731 U.S. PATENT DOCUMENTSExaminer Filing Date If Initial Document Number Date Name Class SubClassAppropriate 4,566,889 01/28/86 Schmadel, Jr. 65 3.2 11/1984 4,593,96906/10/86 J. Goodman et al. 350 96.19 10/28/83 4,629,285 12/16/86 R.Carter et al. 350 96.23 02/21/84 4,725,110 02/16/88 W. Glenn et al. 3503.61 10/27/86 4,807,950 02/28/89 W. Glenn et al. 350 3.61 11/19/874,957,343 09/18/90 T. Sato et al. 350 96.21 10/30/89 4,986,843 01/22/91K. Itoh et al. 65 152 07/11/89 5,013,345 05/07/91 K. Itoh et al. 65 4.212/01/88 5,147,434 09/15/92 K. Itoh et al. 65 12 04/16/91 5,327,51507/05/94 D. Anderson et al. 385 123 01/14/93 5,354,348 10/11/94 T. Zushiet al. 65 423 07/14/92 5,384,884 01/24/95 R. Kashyap et al. 385 12911/08/91 5,400,422 03/21/95 C. Askins et al. 385 37 01/21/93 5,422,74506/06/95 G. Williams et al. 359 3 10/30/92 5,596,669 01/21/97 E. Murphyet al. 385 128 04/21/95 5,620,495 04/15/97 J. Aspell et al. 65 39208/16/95 5,620,496 04/15/97 R. Erdogan et al. 65 425 12/04/95 5,745,61504/28/98 R. Atkins, et al. 385 37 10/11/96 FOREIGN PATENT DOCUMENTSTranslation Document Number Date Country Class SubClass Yes No DE4140087 A1 09.06.93 Germany G02B  6/245 X EP 0715193 A1 05.06.96 EPOG02B  6/245 X EP 0736783 A2 09.10.96 EPO G02B 6/18 X SU 1024861 A23.06.83 Russia G02B 5/16 X OTHER DOCUMENTS (Including Authors, Title,Date, Pertinent Papers, etc.) Sterling, Donald J., “Fiber-Optic Cables”,Technicians Guide to Fiber Optics, Chapter 7, Delmar Publishers, Inc.,1993 G. Meltz, et al., “Formation of Bragg Gratings in Optical Fibers bya Transverse Holographic Method”, 1989 Optical Society of America,OPTICS LETTERS, Vol. 14, No. 15, pp. 823-825 R. J. Campbell and R.Kashyap, “Spectral Profile and Multiplexing of Bragg Gratings inPhotosensitive Fiber”, 1991 Optical Society of America, OPTICS LETTERS,Vol. 16, No. 12, June 15, 1991, pp 898-900 Dingding Tang, et al.,“Annealing of Linear Birefringence in Single-Mode Fiber Coils:Application to Optical Fiber Current Sensors”, Journal of LightwaveTechnology, Vol. 9, No.8 , Aug. 1991, pp 1031-1037 William W. Morey etal., “Photoinduced Bragg Gratings in Optical Fibers”, OPTICS & PHOTONICSNEWS, February 1994, Vol. 5, No.2, pp. 7-14 T. Erdogan, et al., “Decayof Ultraviolet-induced Fiber Bragg Gratings”, Journal of Applied Physics76(1), 1 July 1994, pp. 73-80 Q. Zhang et al., “Tuning Bragg Wavelengthby Writing Gratings on Prestrained Fibers”, IEEE Photonics TechnologyLetters, Vol. 6, No. 7, July 1994, pp. 839- 841 K. C. Byron and H. N.Rourke, “Fabrication of Chirped Fibre Gratings by Novel Stretch andWrite Technique”, ELECTRONICS LETTERS, 5^(th) Jan. 1995, Vol. 31, No. 1,pp. 60-61 N. F. Rizvi and M. C. Gower, “Production of Bragg Gratings inOptical Fibres by Holographic and Mask Projection Methods”, TheInstitution of Electrical Engineers, 1995. M. C. Farries, et al.,“Fabrication and Performance of Packaged Fibre-gratings forTelecommunications”, 1995 Institute of Electrical Engineers, pp. 4/1-4/5EXAMINER Date Considered

What is claimed is:
 1. An apparatus for manufacturing optical fiberBragg gratings, which comprises means for removing a removable coatingfrom at least one predetermined section of an optical fiber element,means for immobilizing the section, means for writing at least one Bragggrating in the section, and means for recoating the section.
 2. Anapparatus as claimed in claim 31, wherein the means for coating removalcomprises at least one of means for mechanically, chemical or thermalremoval.
 3. An apparatus as claimed in claim 32, wherein the means forthermal removal is an air knife.
 4. An apparatus as claimed in claim 31,wherein the means for immobilization comprises a first clamp, a secondclamp, and means for monitoring the strain applied to the optical fiberbetween the first clamp and the second clamp.
 5. An apparatus as claimedin claim 31, wherein the means for writing the grating comprises a laserand a phase mask.
 6. An apparatus as claimed in claim 35, wherein themeans for immobilization further comprises means for rotationallyorienting the predetermined section of the optical fiber element withrespect to a propagation direction of optical radiation emitted by thelaser.
 7. An apparatus as claimed in claim 31, wherein the means forwriting the grating further comprises a grating monitoring systemcomprising a laser, a photodetector, and at least one rotary opticalcoupling which couples the fiber to the laser.
 8. An apparatus asclaimed in claim 31, wherein the means for recoating comprises means forapplying protective coating to the optical fiber, and means for markingthe position.